It is an object of the present invention to provide a newly identified insulin secretion regulation factor that can control blood sugar in the normal range, and a method for screening an antidiabetic drug using said factor. In accordance with the present invention, there is provided a composition which is (i) a protein comprising an amino acid sequence represented by SEQ ID NO: 2 or SEQ ID NO: 4； or (ii) a protein comprising an amino acid sequence in which one or several amino acids are deleted, substituted or added in an amino acid sequence represented by SEQ ID NO: 2 or SEQ ID NO: 4 and negatively regulating insulin secretion, as well as a method for screening antidiabetic drug using said protein or a nucleic acid encoding the same.
